Feedwater preheating system and method

ABSTRACT

The feedwater preheating system includes a feedwater tank and a constant volume recirculation pump. The feedwater tank is adapted to store the feedwater. The feedwater tank includes feed and return lines. The feed line is adapted to feed the feedwater to the HRSG, and the return line enables returning of the feedwater into the feedwater tank. The return line precludes a control valve, and is configured to the feedwater tank to reduce component loss while feedwater recirculation. The constant volume recirculation pump is configured in the feed line to recirculate the feedwater between the HRSG and the feedwater tank. The pump is capable of recirculating the feedwater at constant speed and volume to reduce heat loss while feedwater recirculation, as against the prior art feedwater preheating systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European application 13190995.4 filed Oct. 31, 2013, the contents of which are hereby incorporated in its entirety.

TECHNICAL FIELD

The present disclosure relates to Combined Cycle Power Plants (CCPP), and, more particularly, to feedwater preheating system and method for preheating feedwater to be fed into a Heat Recovery Steam Generator (HRSG) in the CCPP.

BACKGROUND

Where equipped, in Combined Cycle Power Plants (CCPP), feedwater heating systems are generally used to preheat water delivered Heat Recovery Steam Generators (HRSG) to up to or above a fixed minimum temperature required at the HRSG. Preheating the feedwater reduces the irreversibilities involved in steam generation in the HRSG and improves the thermodynamic efficiency of the CCPP. Among various others advantages of the preheating feedwater, such preheating also helps avoiding corrosion caused by flue gas condensation on outer tubes surface in inside the HRSG.

For the CCPP equipped with the feedwater heating systems, the preheating is achieved by different heating source, such as, pegging steam extracted from the HRSG, steam turbine extraction either in a condensate preheater, a separate feedwater preheater, feedwater recirculation from an economizer extraction or recirculation of feedwater in a dedicated HRSG coil at a cold end of the HRSG. A philosophy of the mentioned state of the art concepts is to control a fixed temperature, generally, in the feedwater tank.

A prior art FIG. 4 depicts a typical outline of the feedwater preheating system 10 utilized for preheating and controlling the fixed temperature of the feedwater. The system 10 includes a feedwater tank 12 having a feed line 14 and a return line 16 to feed the preheated water to the HRSG, such as, a HRSG 30. The feed line 14 includes a pump 18 to convey the feedwater to the HRSG 30, and the return line 16 may include a control valve 20 to throttle the flow of the feedwater. The control valve 20 throttles the flow to adjust the recirculation mass flow and therefore adjust the heat input to the feedwater tanks to maintain required temperature. Alternatively, the feedwater flow is controlled by using the pump 18, where the feedwater recirculation mass flow is variable. Nevertheless, for certain load cases of the CCPP, the heat integrated by the feedwater recirculation system operating at full load (e.g. 130 kg/s) may not be enough to maintain the required minimum temperature set point, which may usually be 60° C. or above. For that, as depicted in FIG. 4, the system 10 includes the heating source, such as a pegging steam circuit 22, to supply heated steam to maintain and control the minimum feedwater temperature set point in the feedwater tank 12.

Using any such heating source may constantly affects the CCPP efficiency since the energy used to heat the feedwater is usually derived from a main steam source (CRH, LP, etc.) or extracted between the stages of the steam turbine. Therefore, the steam that would be used to perform expansion work in the turbine (and therefore generate power) is not utilized for that purpose. The percentage of the total cycle steam mass flow used for the feedwater heating must be carefully optimized for maximum CCPP efficiency since increasing in fraction of this steam causes a decrease in the CCPP efficiency.

Accordingly, there exists a need to minimize or preclude the need of utilizing such heating stream to heat the feedwater for heating HRSG to maximize efficiency of the CCPP.

SUMMARY

The present disclosure describes improved feedwater preheating system and method, that will be presented in the following simplified summary to provide a basic understanding of one or more aspects of the disclosure that are intended to overcome the discussed drawbacks, but to include all advantages thereof, along with providing some additional advantages. This summary is not an extensive overview of the disclosure. It is intended to neither identify key or critical elements of the disclosure, nor to delineate the scope of the present disclosure. Rather, the sole purpose of this summary is to present some concepts of the disclosure, its aspects and advantages in a simplified form as a prelude to the more detailed description that is presented hereinafter.

An object of the present disclosure is to describe improved feedwater preheating systems and methods, which may minimize or preclude the need of utilizing high energetics heating stream to heat the feedwater to maximize efficiency of a Combined Cycle Power Plants (CCPP). Such improved feedwater preheating systems and methods may equally be capable in preventing corrosion caused by flue gas condensation on outer tubes surface in inside the HRSG. Further, object of the present disclosure is to describe an improved feedwater preheating systems and methods, which may be convenient to use in an effective and economical way. Various other objects and features of the present disclosure will be apparent from the following detailed description and claims.

The above noted and other objects, in one aspect, may be achieved by an improved feedwater preheating system. The feedwater preheating system is adapted to preheat feedwater to be fed into a Heat Recovery Steam Generator (HRSG). The feedwater preheating system includes a feedwater tank and a constant volume recirculation pump. The feedwater tank is adapted to store the feedwater. The feedwater tank includes a feed line and a return line. The feed line is adapted to feed the feedwater to the HRSG, and the return line adapted to enable returning of the feedwater into the feedwater tank. The return line precludes a control valve to reduce component loss while feedwater recirculation. Further, the constant volume recirculation pump is configured in the feed line to recirculate the feedwater between the HRSG and the feedwater tank. The constant volume recirculation pump recirculate the feedwater at constant speed and volume to reduce heat loss while feedwater recirculation, thereby increasing efficiency of the HRSG.

In one embodiment of the present disclosure, the feedwater is maintained above or at a minimum set temperature, defined during the design based on the sulphur/water content in the flue gas. In such embodiment, preclusion of the control valve from the return line and inclusion of the constant volume recirculation pump, instead of variable volume recirculation pump, enable the feedwater preheating system to recirculate the feedwater at constant speed and volume, reducing heat loss while feedwater recirculation, and maintaining the minimum set temperature of the feedwater, as set during designing.

In further embodiment of the present disclosure, instead of defining the minimum set temperature during the design, as in the above embodiment, the feedwater is maintained at a temperature, a required minimum or above set temperature, as required by the HRSG during operation. As per this embodiment, a Continuous Emission Monitoring System (CEMS) circuit or any other measuring device, such as dedicated water/acid dew point measurement device, is configured to the HRSG to enable calculation of the required minimum set temperature, based on parameters of the HRSG, at which the feedwater is required to be kept for recirculation in the HRSG. In this embodiment, preclusion of the control valve from the return line and inclusion of the constant volume recirculation pump, to recirculate the feedwater at constant speed and volume, reduce heat loss while feedwater recirculation, and maintain the required minimum set temperature of the feedwater, as calculated by the CEMS circuit.

In both the embodiments, the feedwater preheating system includes an optional provision of a pegging steam line source to supply pegging steam to the feedwater tank to preheat the feedwater up to the minimum temperature set during the designing or up to the required minimum set temperature calculated by the CEMS circuit. As and when the temperature of the feedwater in the feedwater tank reaches below the minimum set temperature or below the required calculated minimum set temperature, the pegging steam line source may be activated to attain the feedwater temperature requirement. Such control and reduced use of the pegging steam is capable of increasing efficiency of the HRSG and hence of the CCSS.

Both embodiments of the feedwater preheating system includes a control valve configured in the pegging steam source line to close and open the pegging steam source line to supply the pegging steam. Both the embodiments of the system also further includes a temperature control circuit configured to the control valve of the pegging steam source line and to the feedwater tank, to send signals to open and close the control valve based on the temperature of the feedwater tank in order to maintain the minimum set temperature of the feedwater.

In further aspect of the present disclosure, instead of minimizing the pegging steam for preheating the feedwater, it is complete precluded from the feedwater preheating system. In this aspect, the feedwater preheating system includes a feedwater tank, a constant volume recirculation pump and a pressure control means. The feedwater tank includes a feed line and a return line. The feed line is configured to feed the feedwater to the HRSG; and the return line is configured to enable returning of the feedwater into the feedwater tank. The return line precludes the control valve and is configured to the feedwater tank. Further, the constant volume recirculation pump is configured in the feed line to recirculate the feedwater between the HRSG and the feedwater tank. The constant volume recirculation pump is configured to recirculate the feedwater at constant speed and volume to reduce heat loss while feedwater recirculation. Furthermore, the pressure control means is configured to an evaporator of the HRSG to increase an operational pressure of steam in the evaporator to shift heat to the feed line to increase heat gain in the feedwater and maintain a minimum set temperature of the feedwater, thereby precluding the pegging steam requirement for preheating the feedwater as and when required in the above two embodiments, thereby increasing the efficiency of the HRSG.

The feedwater preheating system may further includes a temperature control circuit configured to the pressure control means of the evaporator and to the feedwater tank, to send signals to increase or decrease the operational pressure of the steam in the evaporator based on the temperature of the feedwater tank in order to maintain the minimum set temperature of the feedwater.

In further aspect, methods are disclosed in the present disclosure to be operable in view of the respective feedwater preheating systems.

Such improved feedwater preheating systems, either minimize or preclude the need of utilizing heating stream to heat the feedwater for heating a Heat Recovery Steam Generator (HRSG) to maximize efficiency of a Combined Cycle Power Plants (CCPP). Such improved feedwater preheating systems and methods may equally be capable in preventing corrosion caused by flue gas condensation on outer tubes surface in inside the HRSG. Further, the improved feedwater preheating systems may be convenient to use in an effective and economical way.

These together with the other aspects of the present disclosure, along with the various features of novelty that characterize the present disclosure, are pointed out with particularity in the present disclosure. For a better understanding of the present disclosure, its operating advantages, and its uses, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated exemplary embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features of the present disclosure will be better understood with reference to the following detailed description and claims taken in conjunction with the accompanying drawing, wherein like elements are identified with like symbols, and in which:

FIG. 1 illustrates an example line diagram of a feedwater preheating system, in accordance with first exemplary embodiment of the present disclosure; and

FIG. 2 illustrates an example line diagram of a feedwater preheating system, in accordance with second exemplary embodiment of the present disclosure;

FIG. 3 illustrates an example line diagram of a feedwater preheating system, in accordance with third exemplary embodiment of the present disclosure; and

FIG. 4 illustrates an example line diagram of a feedwater preheating system as per prior art.

Like reference numerals refer to like parts throughout the description of several views of the drawings.

DETAILED DESCRIPTION

For a thorough understanding of the present disclosure, reference is to be made to the following detailed description, including the appended claims, in connection with the above described drawings. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be apparent, however, to one skilled in the art that the present disclosure can be practiced without these specific details. In other instances, structures and apparatuses are shown in block diagrams form only, in order to avoid obscuring the disclosure. Reference in this specification to “one embodiment,” “an embodiment,” “another embodiment,” “various embodiments,” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. The appearance of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but may not be of other embodiment's requirement.

Although the following description contains many specifics for the purposes of illustration, anyone skilled in the art will appreciate that many variations and/or alterations to these details are within the scope of the present disclosure. Similarly, although many of the features of the present disclosure are described in terms of each other, or in conjunction with each other, one skilled in the art will appreciate that many of these features can be provided independently of other features. Accordingly, this description of the present disclosure is set forth without any loss of generality to, and without imposing limitations upon, the present disclosure. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

Referring now to FIGS. 1 to 3, various examples of feedwater preheating systems illustrated in accordance with various exemplary embodiments of the present disclosure. In as much as the construction and arrangement of the feedwater preheating systems and its arrangement with respect a Heat Recovery Steam Generator (HRSG) 200 to maximize efficiency of a Combined Cycle Power Plants (CCPP), various associated elements may be well-known to those skilled in the art, it is not deemed necessary for purposes of acquiring an understanding of the present disclosure that there be recited herein all of the constructional details and explanation thereof. Rather, it is deemed sufficient to simply note that as shown in FIGS. 1 to 3, the feedwater preheating systems 100, only those components are shown that are relevant for the description of various embodiments of the present disclosure.

As shown in FIG. 1, according to first aspect the present disclosure, the feedwater preheating system 100 includes a feedwater tank 110 and a constant volume recirculation pump 120. The feedwater tank 110 is adapted to store the feedwater. The feedwater tank 110 includes a feed line 112 and a return line 114. The feed line 112 is adapted to feed the feedwater to the HRSG 200. Further the return line 114 is adapted to enable returning of the feedwater into the feedwater tank 110. As compared to the prior art feedwater preheating system 10, shown in FIG. 4, the return line 114 of the present embodiment precludes a control valve 20, and that the return line 114 is directly configured to the feedwater tank 110 to reduce component loss while feedwater recirculation. Further, the constant volume recirculation pump 120 is configured in the feed line 112 to recirculate the feedwater between the HRSG 200 and the feedwater tank 110. The constant volume recirculation pump 120 is fully redundant to recirculate the feedwater at constant speed and volume to reduce heat loss while feedwater recirculation, as against the prior art feedwater preheating system 10 that recirculate feedwater with variable speed and volume.

In one embodiment of the present disclosure as shown in FIG. 1, the feedwater is maintained at minimum or above a set temperature, defined during the design based on the sulphur/water content in the flue gas of the CCPP. In this embodiment, preclusion of the control valve 10 (as shown in prior art FIG. 4) from the return line 114 and inclusion of the constant volume recirculation pump 120 instead of the variable speed and volume pumps 18 (as shown in prior art FIG. 4), make the feedwater preheating system 100 fully redundant to recirculate the feedwater at constant speed and volume, reduces heat loss while feedwater recirculation and maintain the minimum set temperature of the feedwater, as set during designing.

In further embodiment of the present aspect as shown in FIG. 1, the feedwater preheating system 100 may further include an optional provision of a pegging steam line source 130 to supply a pegging steam in the feedwater tank 110 to reheat the feedwater to up to the minimum set temperature set during the designing. As and when the temperature of the feedwater in the feedwater tank 110 reaches below the minimum set temperature, the pegging steam line source 130 may be activated to attain the feedwater temperature requirement. Such control and reduced use of the pegging steam is capable of increasing efficiency of the HRSG and hence of the CCSS.

In further embodiment, the feedwater preheating system 100, as shown in FIG. 1, includes a control valve 140 configured in the pegging steam source line 130 to close and open the pegging steam source line 130 to supply the pegging steam as and when required. Further, the system 100 also includes a temperature control circuit 150 configured to the control valve 140 of the pegging steam source line 130 and to the feedwater tank 110, to send signals to open and close the control valve 140 based on the temperature of the feedwater tank 110 in order to maintain the minimum set temperature of the feedwater.

Referring now to FIG. 2, a second aspect of the present disclosure, where instead of defining the minimum set temperature during the design, as in the first aspect described with respect to FIG. 1, the feedwater is maintained at or above a required temperature, herein after referred as “required minimum temperature,” as required by the HRSG 200 during the operation. As per this aspect, shown in FIG. 2, a Continuous Emission Monitoring System (CEMS) circuit 160 is configured to the HRSG 200 to calculate the required minimum temperature for the feedwater, based on parameters of the HRSG 200., at which the feedwater is required to be kept for recirculation in the HRSG 200 to avoid corrosion thereof. However, without departing from the scope of the present disclosure, apart from the CEMS, any other measuring device, such as dedicated water/acid dew point measurement device may also be used to for calculation of the required minimum temperature. Further, in this second aspect (as shown in FIG. 2), similar to the first aspect (as per FIG. 1), preclusion of the control valve 10 from the return line 114 and the constant volume recirculation pump 120, makes the feedwater preheating system 100 fully redundant to recirculate the feedwater at constant speed and volume, reduce heat loss while feedwater recirculation, and maintain the required minimum temperature of the feedwater, as calculated by the CEMS circuit 160.

Similar to the first aspect (shown in FIG. 1), the second aspect (shown in FIG. 2) of the feedwater preheating system 100 also includes the optional provision of the pegging steam line source 130 to supply the pegging steam in the feedwater tank 110 to reheat the feedwater to up to the required minimum temperature calculated by the CEMS circuit 160. As and when the temperature of the feedwater in the feedwater tank 110 reaches below the required calculated minimum temperature, the pegging steam line source 130 may be activated to attain the feedwater temperature requirement. Such control and reduced use of the pegging steam is capable of increasing efficiency of the HRSG 200 and in turn the efficiency of the CCSS.

Similar to first aspect as shown in FIG. 1, the feedwater preheating system 100 of the second aspect (shown in FIG. 2), also includes the control valve 140 configured in the pegging steam source line 130 to close and open the pegging steam source line 130 to supply the pegging steam as and when required. Further, the system 100 of second aspect also includes the temperature control circuit 150 configured to the control valve 140 of the pegging steam source line 130 and to the feedwater tank 110, to send signals to open and close the control valve 140 based on the temperature of the feedwater tank 110 in order maintain the required minimum temperature of the feedwater calculated by the CEMS circuit 160.

Referring now to FIG. 3, third aspect of the feedwater preheating system 100 is depicted. As per this aspect, the feedwater preheating system 100, instead of minimizing the pegging steam for preheating the feedwater which may optionally be required in the feedwater preheating system 100 of the first and second aspects, such use of the pegging steam is completely precluded. In this aspect, the feedwater preheating system 100 includes a feedwater tank 110, a constant volume recirculation pump 120 and a pressure control means 170. The feedwater tank 110 includes a feed line 112 and a return line 114, as similar to the first and second aspects of the feedwater preheating system 100, and excluded herein from explanation for the sake of brevity. Further, the constant volume recirculation pump 130 of the third aspect is similar to the first and second aspects of the feedwater preheating system 100, and excluded herein from explanation for the sake of brevity. The third aspect of the feedwater preheating system 100 includes the pressure control means 170, which is configured at a steam turbine inlet, specifically to an evaporator 210 of the HRSG 200, to increase an operational pressure of steam in the evaporator 210 to shift heat to the feed line 112 to increase heat gain in the feedwater and maintain the minimum set temperature of the feedwater. Since the heat required for preheating the feedwater to up to the minimum set temperature is gained by the evaporator 210 by shifting its heat to the feed line 112, the pegging steam requirement for preheating the feedwater as and when required in the above two aspects is excluded in the third aspect of the present disclosure, thereby increasing the efficiency of the HRSG 200.

The feedwater preheating system 100 as shown in FIG. 3 may include a temperature control circuit 180 configured to the pressure control means 170 of the evaporator 210 and to the feedwater tank 110, to send signals to increase or decrease the operational pressure of the steam in the evaporator 210 based on the temperature of the feedwater tank 110 in order to maintain the minimum set temperature of the feedwater.

The invention of the present disclosure is advantageous in various scopes. Such improved feedwater preheating systems, either minimize or preclude the need of utilizing heating stream to heat the feedwater for heating a Heat Recovery Steam Generator (HRSG) to maximize efficiency of a Combined Cycle Power Plants (CCPP). Such improved feedwater preheating systems and methods may equally be capable in preventing corrosion caused by flue gas condensation on outer tubes surface in inside the HRSG. Further, the improved feedwater preheating systems may be convenient to use in an effective and economical way. Various other advantages and features of the present disclosure are apparent from the above detailed description and appendage claims.

The foregoing descriptions of specific embodiments of the present disclosure have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present disclosure to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present disclosure and its practical application, to thereby enable others skilled in the art to best utilize the present disclosure and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present disclosure. 

What is claimed is:
 1. A feedwater preheating system for preheating feedwater to be fed into a Heat Recovery Steam Generator (HRSG), the feedwater preheating system, comprising: a feedwater tank to store the feedwater, the feedwater tank having a feed line to feed the feedwater to the HRSG, and a return line to enable returning of the feedwater into the feedwater tank, wherein the return line precludes a control valve and is configured to the feedwater tank to reduce component loss while feedwater recirculation; and a constant volume recirculation pump configured in the feed line to recirculate the feedwater between the HRSG and the feedwater tank, wherein the constant volume recirculation pump is capable of recirculating the feedwater at constant speed and volume to reduce heat loss while feedwater recirculation, thereby increasing efficiency of the HRSG.
 2. The feedwater preheating system as claimed in claim 1, wherein the feedwater is maintained at or above a minimum set temperature.
 3. The feedwater preheating system as claimed in claim 2, further comprising a pegging steam source line configured to the feedwater tank to supply pegging steam to preheat the feedwater as and when the temperature of the feedwater in the feedwater tank reaches below the minimum set temperature.
 4. The feedwater preheating system as claimed in claim 3, further comprising a control valve configured in the pegging steam source line to close and open the pegging steam source line to supply the pegging steam.
 5. The feedwater preheating system as claimed in claim 4, further comprising a temperature control circuit configured to the control valve of the pegging steam source line and to the feedwater tank, to send signals to open and close the control valve based on the temperature of the feedwater tank in order to maintain the minimum set temperature of the feedwater.
 6. The feedwater preheating system as claimed in claim 1, wherein the feedwater is maintained at a temperature, a required minimum temperature, as required by the HRSG.
 7. The feedwater preheating system as claimed in claim 6 further comprising a pegging steam source line configured to the feedwater tank to supply pegging steam to preheat the feedwater as and when the temperature of the feedwater in the feedwater tank reaches below the required minimum temperature.
 8. The feedwater preheating system as claimed in claim 7 further comprising a Continuous Emission Monitoring System (CEMS) circuit configured to the pegging steam source line and to the HRSG to enable calculation of the required minimum temperature, based on parameters of the HRSG, at which the feedwater is required to be kept for recirculation in the HRSG, to enable the pegging steam source line to supply the pegging steam in the feedwater tank to reheat the feedwater to the required minimum temperature calculated by the CEMS circuit.
 9. The feedwater preheating system as claimed in claim 8, further comprising a control valve configured in the pegging steam source line to close and open the pegging steam source line to supply the pegging steam.
 10. The feedwater preheating system as claimed in claim 9, further comprising a temperature control circuit configured to the control valve of the pegging steam source line and to the feedwater tank, to send signals to open and close the control valve based on the temperature of the feedwater tank in order to maintain the required minimum temperature of the feedwater.
 11. A feedwater preheating system for preheating feedwater to be fed into a Heat Recovery Steam Generator (HRSG), the feedwater preheating system, comprising: a feedwater tank to store the feedwater, the feedwater tank having a feed line to feed the feedwater to the HRSG, and a return line to enable returning of the feedwater into the feedwater tank, wherein the return line precludes a control valve and is configured to the feedwater tank; and a constant volume recirculation pump configured in the feed line to recirculate the feedwater between the HRSG and the feedwater tank, wherein the constant volume recirculation pump is capable of recirculating the feedwater at constant speed and volume to reduce heat loss while feedwater recirculation, and maintain a minimum set temperature of the feedwater.
 12. The feedwater preheating system as claimed in claim 11, further comprising a pegging steam source line configured to feedwater tank to supply pegging steam to preheat the feedwater as and when the temperature of the feedwater in the feedwater tank 110 reaches below the minimum set temperature.
 13. The feedwater preheating system as claimed in claim 12, further comprising a control valve configured in the pegging steam source line to close and open the pegging steam source line to supply the pegging steam.
 14. The feedwater preheating system as claimed in claim 13, further comprising a temperature control circuit configured to the control valve of the pegging steam source line and to the feedwater tank, to send signals to open and close the control valve based on the temperature of the feedwater tank in order to maintain the minimum set temperature of the feedwater.
 15. A feedwater preheating system for preheating feedwater to be fed into a Heat Recovery Steam Generator (HRSG), the feedwater preheating system, comprising: a feedwater tank to store the feedwater, the feedwater tank having a feed line to feed the feedwater to the HRSG, and a return line to enable returning of the feedwater into the feedwater tank, wherein the return line precludes a control valve and is configured to the feedwater tank; a constant volume recirculation pump configured in the feed line to recirculate the feedwater between the HRSG and the feedwater tank, wherein the constant volume recirculation pump is capable of recirculating the feedwater at constant speed and volume to reduce heat loss while feedwater recirculation; a pegging steam source line configured to feedwater tank to supply pegging steam to preheat the feedwater; and a Continuous Emission Monitoring System (CEMS) circuit configured to the pegging steam source line and to the HRSG, to enable calculation of a required minimum temperature at which the feedwater is required to be kept for recirculation in the HRSG, based on parameters of the HRSG, and to enable the pegging steam source line to supply the pegging steam in the feedwater tank to reheat the feedwater to the required minimum temperature calculated by the CEMS circuit.
 16. The feedwater preheating system as claimed in claim 15, further comprising a control valve configured in the pegging steam source line to close and open the pegging steam source line to supply the pegging steam as and when the temperature of the feedwater in the feedwater tank reaches below the required minimum temperature.
 17. The feedwater preheating system as claimed in claim 16, further comprising a temperature control circuit configured to the control valve of the pegging steam source line and to the feedwater tank, to send signals to open and close the control valve based on the temperature of the feedwater tank in order to maintain the required minimum temperature of the feedwater.
 18. A feedwater preheating system for preheating feedwater to be fed into a Heat Recovery Steam Generator (HRSG), the feedwater preheating system, comprising: a feedwater tank to store the feedwater, the feedwater tank having a feed line to feed the feedwater to the HRSG, and a return line to enable returning of the feedwater into the feedwater tank, wherein the return line precludes a control valve and is configured to the feedwater tank; a constant volume recirculation pump configured in the feed line to recirculate the feedwater between the HRSG and the feedwater tank, wherein the constant volume recirculation pump is capable of recirculating the feedwater at constant speed and volume to reduce heat loss while feedwater recirculation; and a pressure control means configured to a steam turbine inlet at an evaporator of the HRSG to increase an operational pressure of steam in the evaporator to shift heat to the feed line to increase heat gain in the feedwater and maintain a minimum set temperature of the feedwater, thereby increasing the efficiency of the HRSG.
 19. The feedwater preheating system as claimed in claim 18, further comprising a temperature control circuit configured to the pressure control means of the evaporator and to the feedwater tank, to send signals to increase or decrease the operational pressure of the steam in the evaporator based on the temperature of the feedwater tank in order to maintain the minimum set temperature of the feedwater.
 20. A method for preheating feedwater to be fed into a Heat Recovery Steam Generator (HRSG), the method comprising: circulating the feedwater in a feed line and a return line of a feedwater tank at constant speed and volume to reduce heat loss while feedwater recirculation to maintain a minimum set temperature of the feedwater, wherein the return line precludes a control valve and is configured to the feedwater tank to reduce component loss while feedwater recirculation, thereby increasing the efficiency of the HRSG.
 21. The method as claimed in claim 20, further comprising supplying pegging steam to preheat the feedwater as and when the temperature of the feedwater reaches below the minimum set temperature.
 22. A method for preheating feedwater to be fed into a Heat Recovery Steam Generator (HRSG), the method comprising: circulating the feedwater in a feed line and a return line of a feedwater tank at constant speed and volume to reduce heat loss while feedwater recirculation, wherein the return line precludes a control valve and is configured to the feedwater tank to reduce component loss while feedwater recirculation; calculating a required set temperature of the HRSG at a last stage; and supplying pegging steam to preheat the feedwater in the feedwater tank as and when the temperature of the feedwater reaches below the required minimum temperature.
 23. A method for preheating feedwater to be fed into a Heat Recovery Steam Generator (HRSG), the method comprising: circulating the feedwater in a feed line and a return line of a feedwater tank at constant speed and volume to reduce heat loss while feedwater recirculation, wherein the return line precludes a control valve and is configured to the feedwater tank to reduce component loss while feedwater recirculation; and increasing an operational pressure of steam in an evaporator of the HRSG to shift heat to the feed line to increase heat gain in the feedwater and maintain a minimum set temperature of the feedwater, thereby increasing the efficiency of the HRSG. 